This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The fast uptake of the Third Generation Partnership Project (3GPP)-Long Term Evolution (LTE) in different regions of the world shows both that demand for wireless broadband data is increasing, and that LTE is a successful platform to meet that demand. The existing and new spectrum licensed for exclusive use by International Mobile Telecommunications (IMT) technologies will remain fundamental for providing seamless coverage, achieving the higher spectral efficiency, and ensuring the higher reliability of cellular networks through careful planning and deployment of high-quality network equipment and devices.
In order to meet the ever increasing data traffic demand from users, particularly in concentrated high traffic buildings or hot spots, more mobile broadband bandwidth will be needed. Given a large amount of spectrum available in unlicensed bands around the globe, the unlicensed spectrum is more and more considered by cellular operators as complementary means to augment their service offering. While the unlicensed spectrum may not match the qualities of the licensed regime, solutions that allow an efficient use of it as a complement to licensed deployments have a potential to bring a great value to 3GPP operators, and ultimately to the 3GPP industry as a whole. This type of solutions would enable operators and vendors to leverage the existing or planned investments in LTE/Evolved Package Core (EPC) hardware in radio and core networks.
Licensed-Assisted Access
It has been agreed to study Licensed-Assisted Access (LAA) technologies in the 3GPP at RP-141664. This LAA framework builds on carrier aggregation solutions introduced in LTE Release-10 to access the additional bandwidth in the unlicensed spectrum. FIG. 1 shows an illustrative LTE network that configures a user equipment, UE, to aggregate additional secondary cells (SCells) which are using frequency carriers in the unlicensed spectrum. The primary cell (PCell) maintains exchange of essential control messages and also offers an always-available robust spectrum, i.e. the licensed spectrum, for real-time and high-value traffic. Via the high-quality and robust licensed spectrum, the PCell may also provide mobility handling and management for the UE. The aggregated SCells in the unlicensed spectrum, when available, can be utilized as a bandwidth booster to serve, e.g. the best effort traffic. The LAA SCells may operate in a downlink (DL)-only mode or operate with both uplink (UL) and DL traffic.
DRX Scheme in LTE
The 3GPP specifications for LTE, e.g. 3GPP TS 36.321 v11.5.0, specify a procedure for discontinuous reception (DRX). A UE may be configured by RRC with a DRX functionality that controls the UE's Physical Downlink Control CHannel (PDCCH) monitoring activity. When in RRC_CONNECTED, if DRX is configured, the UE is allowed to monitor the PDCCH discontinuously using the DRX operation as specified in the specification; otherwise the UE monitors the PDCCH continuously. RRC controls DRX operation by configuring the timers, such as onDurationTimer, drx-InactivityTimer, drx-RetransmissionTimer, and the longDRX-Cycle, and optionally the drxShortCycleTimer and shortDRX-Cycle etc. The onDurationTimer specifies the number of consecutive PDCCH-subframe(s) at the beginning of a DRX Cycle. The drx-InactivityTimer specifies the number of consecutive PDCCH-subframe(s) after the subframe in which a PDCCH indicates an initial Uplink (UL) or Downlink (DL) user data transmission for this UE. The drx-RetransmissionTimer specifies the maximum number of consecutive PDCCH-subframe(s) until a DL retransmission is received. The DRX Cycle specifies the periodic repetition of the On Duration followed by a possible period of inactivity, comprising the long DRX-Cycle and the shortDRC-Cycle. The drxShortCycleTimer specifies the number of consecutive subframe(s) the UE shall follow the short DRX cycle.
FIG. 2 shows a basic DRX procedure in LTE. As illustrated, the blocks with oblique lines represent the active time defined by the onDurationTimer or the drx-InactivityTimer, while the blank blocks represent the time that no transmission occurs. During the first two longDRX-Cycles, no DL/UL grant is received in the active time defined by the onDurationTimer. During the third longDRX-Cycle, a DL/UL grant is received in the active time defined by the onDurationTimer, which is represented with a dotted block in FIG. 2, and then the drx-InactivityTimer is started. During the active time defined by the drx-InacitivityTimer, the UE can conduct continuous transmission. If the drx-InactivityTimer expires or a DRX Command MAC control element is received, the drxShortCycleTimer starts and the short DRX cycle will be used.
Powering Saving Scheme in 802.11
IEEE-802.11 was designed with power saving in mind for terminal stations. To assist terminal stations with power saving, Access Points (APs) are designed to buffer frames for a terminal station when that terminal station is in power save mode and to transmit them later to the terminal station when the AP knows the terminal station will listen. When a terminal station is in power save mode, it turns off its transmitter and receiver to preserve energy. It takes less power for a terminal station to turn its receiver on to listen to frames than to turn its transmitter on to transmit frames. For this reason, it's more power-efficient for an AP to inform a terminal station if it has buffered frames present on the AP than to have the terminal station poll the AP querying if frames are present.
Target Beacon Transmission Time (TBTT) is the time at which a node (AP or terminal station when in Ad-hoc) must send a beacon. The time difference between two TBTTs is known as the beacon interval. The beacon interval is given in Time Units (TU), each TU represents 1024 microseconds. The beacon interval is typically set to 100 TUs (102400 microseconds, or 102.4 ms) and its length is two bytes.
During association, a “Listen Interval” field is provided by the terminal station. The listen interval is given in beacon interval units, so essentially it tells the AP how many beacons it wants to ignore before turning the receiver on. Two bytes are used to represent the listen interval. Depending on how the AP is tuned, usually based on the amount of space desired for buffered frames, the association may or may not be rejected. After the listen interval, the AP does not guarantee it will buffer frames for the terminal station anymore and may discard them. As might be expected then, the listen interval can be used by the AP as a guaranteed maximum time before terminal stations listen to one of their beacons.
The IEEE-802.11 standards chose to use a bitmap to indicate to any sleeping listening terminal stations if the AP has any buffered frames present for it. Because terminal stations should listen to at least one beacon before the listen interval, the AP periodically sends this bitmap on its beacons as an information element. The bit mask is called the Traffic Indication Map and consists of 2008 bits, each bit representing the Association Id (AID) of a terminal station.
After a terminal station receives a Traffic Indication Map (TIM) and if it sees that the AP has buffered frames for it, it must send a Power Save Poll (PS-Poll) control frame to retrieve each buffered frame on the AP. The terminal station may go back to sleep after the PS-Poll frame exchange or once the TIM no longer has its AID present.
In an 802.11 Wireless Local Area Network (WLAN) system, beacon transmission also follows the Carrier Sense Multiple Access with Collision Detection (CSMA/CA) scheme, which is also called Listen Before Talk (LBT). LBT is a technique used in radio communications whereby a radio transmitters first senses its radio environment before it starts a transmission. LBT can be used by a radio device to find a network the device is allowed to operate on or to find a free radio channel to operate on.
However, direct application of the existing DRX scheme in LTE or power saving scheme in 802.11 to LAA networks may cause some problems due to involvement of both licensed carriers and unlicensed carriers into cellular transmission.